Ejection head for aggressive liquids manufactured by anodic bonding

ABSTRACT

A method for manufacturing an ejection head ( 10 ) or ejector suitable for ejecting in the form of droplets ( 16 ) a liquid ( 14 ) conveyed inside the ejection head ( 10 ), comprising a step of producing, from a silicon wafer, a nozzle plate ( 12 ) having at least one ejection nozzle ( 13 ); a step of producing, from another silicon wafer, a substrate ( 11 ) having at least one actuator ( 15 ) for activating the ejection of the droplets of liquid through the nozzle ( 13 ); and a step of joining the nozzle plate ( 12 ) and the substrate ( 11 ) together to form the ejection head, wherein this joining step comprises the production of a junction ( 25 ), made by means of the anodic bonding technology, between the substrate ( 11 ) and the nozzle plate ( 12 ), in such a way that, in the area of this junction ( 25 ), the ejection head ( 10 ) does not present structural discontinuities, and also possesses a resistance to chemical corrosion by the liquid ( 14 ) contained in the ejection head ( 10 ) at least equal to that of the silicon constituting both the substrate ( 11 ) and the nozzle plate ( 12 ). The method of the invention may be applied for manufacturing an ink jet printhead ( 110 ), having one or more nozzles ( 113   a,    113   b , etc.), which has the advantage, with respect to the known printheads, of also being suitable for working with special inks characterized by high level chemical aggressiveness. In general, the ejection head of the invention, thanks to its structure which is globally highly robust and also chemically inert in the area of the junction ( 25 ), can be used advantageously with various types of liquids, even with marked chemical aggressiveness, in different sectors of the art, for example for ejecting paints on various types of media, generally not paper, in the industrial marking sector; or for ejecting in a controlled way droplets of fuel, such as petrol, in an internal combustion engine.

This is a U.S. National phase Application Under 35 USC 371 and applicantherewith claims the benefit of priority of PCT/IT01/00266 filed May 25,2001, which was published Under PCT Article 21(2) in English andApplication No. T02000A000494 filed in Italy on May 29, 2000.

TECHNICAL FIELD

This invention relates in general to the sector of ejection heads forejecting liquids in the form of droplets, and in particular to anejection head provided with a structure that makes this ejection headhighly suited to working with liquids having a high level of chemicalaggressiveness.

The invention also relates to a method for manufacturing an ejectionhead provided with a special resistance to chemically highly aggressiveliquids, so as to be able to be employed advantageously in combinationwith this category of liquids.

BACKGROUND ART

The ejection head, also called simply ejector or injector in thefollowing, according to the invention has characteristics that render itadvantageous for use in numerous industrial sectors, even withspecifics, characteristics and problems differing completely from onesector to the next.

In particular, among the possible sectors of application are, purely byway of example, that of ink jet printing, or that of fuel injection inan internal combustion engine.

As will be clear in the remainder of the description, the ejection headof the invention presents significant similarities, both structural andoperational, with a thermal ink jet printhead, of the type working onthe basis of the so-called bubble ink jet printing technology.Printheads of this type are widely known in the sector of ink jetprinting technologies, where they are applied in a variety of solutions,and are still undergoing significant developments.

Therefore, for the sake of completeness and in order to facilitate theunderstanding of this description, and also in consideration of the factthat the ink jet printing sector constitutes, as already said, one ofthe possible and main fields of application of this invention, thegeneral characteristics of these bubble type thermal ink jet printheadsand some of their most recent developments will be set down in shortbelow. As is known, in the printheads working with the bubble type inkjet technology, the ink contained in the printhead is brought to boilingpoint by thermal actuators consisting of electrical resistances whichare powered with opportune current pulses in order to activate, insidethe ink, the appearance of a bubble of vapour which, by expanding,causes ejection of the droplets through a plurality of nozzles in theprinthead.

The printheads operating with the bubble technology may be divided intotwo main categories, depending on their structure, called respectively“top shooter” and “edge shooter”. In the first type, the nozzle consistsof an aperture arranged immediately above the thermal actuator andseparated from the latter by a small chamber filled with ink, so thatthe expansion of the bubble of vapour is used in a directionperpendicular to the thermal actuator so as to eject the droplet throughthe aperture. In the second type, the thermal actuator is disposed alongthe wall of a duct a short way from the duct's outlet section to theoutside, so that the expansion of the bubble of steam is used in adirection transversal to the actuator to eject the drop laterallythrough the outlet section of the duct.

This bubble technology has been a standard in the printing sector formany years now, and is applied with success on numerous models of inkjet printheads, both for black and white printing and for colourprinting. In particular, the ink jet printheads that work according tothis technology are moving towards ever greater levels of integrationand complexity, the objective being to comprise a greater number ofcircuits, nozzles and functions, and therefore attain ever greaterprinting speeds and definitions. One of the most recent examples of thistechnical development is represented by what are known as the monolithicprintheads, i.e. by thermal ink jet heads in which the nozzle plate ismade, not as a separate part, but together with the other parts of theprinthead, particularly with those parts that constitute the drivercircuits of the actuators and the hydraulic network for conveying theink inside the printhead.

Therefore in these monolithic heads, the nozzle plate does notconstitute a piece which is made separately and mounted at the end ofthe process of manufacturing the printheads, but rather a part which isformed progressively in the manufacturing process, so that eachprinthead acquires a typically monolithical structure integrating thevarious parts.

Hand in hand with the constant evolution of the bubble ink jet thermalprintheads, the inks that can be used on these heads have also evolvedconsiderably, which has led to a continuous improvement in their qualityand reliability.

Generally speaking, evolution of the printheads has been accompanied bya corresponding evolution of the inks, the objective being to researchever better combinations between the printing media intended forreceiving the droplets of ink, the structural characteristics of thehead, and the chemical characteristics of the inks.

Typically this research into inks has been conducted with the objectiveof formulating inks capable both of improving the print quality on anever broader range of print media, and of mating optimally with the newstructures of printheads brought out with time.

In this way, both black and coloured inks have been formulated capableof minimizing the problem of clogging of the nozzles, cause bysedimentation of the pigments contained in the inks, despite the evermore intense miniaturization of the printheads and the reduction of thediameter of the nozzles in order to obtain ever smaller droplets.

Additionally, the research has permitted to define optimal combinationsbetween inks and materials used in manufacturing the heads, with inksand materials compatible with one another, i.e. capable of nottriggering off undesired reactions, and of maintaining their nominalcharacteristics in time, so as not to have negative effects on theoperation and reliability of the printheads. In particular, thisresearch into, as stated, constantly improving the combination betweeninks, print media, and printheads, has obviously addressed theformulation of inks having a low or practically null degree of chemicalaggressiveness, namely inks free of substances capable of aggressing,corroding and reacting with, even only minimally, the various materialsemployed in manufacturing the heads and wetted by the inks.

For instance, it was attempted to avoid those inks containing substancesthat could interact with the organic compounds usually employed inmaking the junctions between the parts of the head. However, in thisway, recent research in inks has in fact resulted in a certainconsolidation, regarding their use on printheads, of inks with a null orpractically null level of chemical aggressiveness.

At the same time, the possibility was ignored of employing theseprintheads in combination with particular types of ink and/or in generalliquids which, though widely applied and capable of giving optimalresults in certain fields, including different from printing true andproper, possessed however characteristics of chemical aggressivenessincompatible with the structure of the printheads that were beingdeveloped, and in particular contained aggressive substances certainlycapable of corroding them and compromising their operation in time.

Besides, as is easy to imagine, it could be very useful and advantageousto be able to dispose of a new ink jet printhead, of the type based onthe bubble technology or also on other technologies, having the abilityto work with inks, perhaps already employed with success in variousapplications, including different from printing on paper, butunfortunately containing corrosive and/or aggressive substances likelyto damage in time the structure and the materials of the currently knownbubble type thermal ink jet heads. In fact, in this way the applicationpossibilities for these printheads could be considerably extended,considering the new properties, essential characteristics andperformance advantages that these corrosive substances could confer onthe inks employed with them. Unfortunately however, as said, in realitythe known ink jet printheads do not have a structure capable ofresisting corrosive agents that may possibly be present in the inksemployed with the printheads, so that in this hypothetical case theywould rapidly enter decay.

For example, as is known, inks known to be typically aggressive,containing for instance urea, and/or having a determined acidic PH, cancertainly not be used on the current thermal heads, because they wouldsurely damage the junctions and the gluing zones between the differentlayers comprising the structure of the head.

There are also sectors in the art, again completely different from thatof ink jet printing and the relative printheads, in which it isnecessary to eject liquids in the form of droplets, preferably also verysmall, and in which these liquids to be ejected are particularlyaggressive from the chemical viewpoint, and at any rate have acomposition incompatible with the structure of the currently knownprintheads

An important one of these sectors, briefly hinted at above, is that ofthe injection of a fuel, such as diesel or petrol, in the combustionchamber of an internal combustion engine. In this sector, the solutionsnormally adopted for fuel injection are based on mechanical typeinjectors, which however have the disadvantage of not reaching asufficient degree of miniaturization of the droplets, or to put itbetter, that degree of miniaturization which would allow a better andmore precise dosage of the fuel, and accordingly to attain betterperformance of the engine, such as for instance a higher thermalefficiency.

Therefore, potentially at least, this sector could avail of the ink jettechnology which, in comparison with the traditional fuel ejectors, hasbeen shown capable of obtaining droplets of liquid much smaller involume, as also of obtaining in general a better and more efficientcontrol of the quantity of liquid ejected in droplet form.

Yet another sector where there may be the need to dose in a precise andcontrolled way particularly aggressive liquids from the chemicalviewpoint is the biomedical sector.

DISCLOSURE OF THE INVENTION

The general object, therefore, of this invention is to produce a newejection head which, though bearing some similarities to the known inkjet printheads, substantially innovates with respect to the latter, andin particular possesses characteristics likely to make its use possibleand advantageous in combination with particularly aggressive liquidsfrom a chemical viewpoint, including in industrial sectors highlydifferent from ink jet printing, and for example in the sector ofinjection of fuel in an internal combustion engine.

This object is achieved by the ejection head and correspondingmanufacturing method having the characteristics defined in the mainindependent claims.

A more specific object of this invention is to produce an ink jetprinthead, of the type operating with the bubble technology or othertechnologies, that can be used without drawbacks with aggressive inksnotoriously capable of chemically reacting with and/or corroding thematerials, typically organically based ones, currently used in themanufacture of printheads, so as to allow, at least potentially, anextension of the possibilities of industrial application of thetechnologies and concepts developed in connection with the knownprintheads to sectors up till now excluded from these technologies andconcepts.

These and other objects, characteristics and advantages of the inventionwill be apparent from the description that follows of a preferredembodiment, provided purely by way of an illustrative, non-restrictiveexample, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1—is a schematic, sectional view of a head for the ejection ofdroplets of liquid according to this invention;

FIG. 2—is a synthetic flow diagram of a method according to thisinvention for manufacturing the ejection head of FIG. 1;

FIG. 3—(section a-g), comprising FIG. 3a and FIG. 3b, is a sectionalview illustrating in sequence the various steps for manufacturing aplate with nozzle of the ejection head of FIG. 1;

FIG. 4—(section a-c) is a sectional view illustrating the final stepsfor making the structure of a substrate bearing an actuator of theejection head of FIG. 1;

FIG. 5—is a working diagram relating to a mounting operation, performedby means of the “anodic bonding” type technology, for soldering thenozzle plate of FIG. 3 to the substrate of FIG. 4;

FIG. 6—shows a first example of application of the invention concerninga printhead provided with multiple nozzles and suitable for ejectingdroplets of ink;

FIG. 7—illustrates a silicon wafer used for manufacturing a plurality ofnozzle plates of the printhead of FIG. 6;

FIG. 8—illustrates another silicon wafer used for manufacturing aplurality of substrates of the printhead of FIG. 6; and

FIG. 9—demonstrates a second example of application of the ejection headmade with the method of the invention, in which the ejection head isarranged for ejecting droplets of fuel in an internal combustion heatengine.

BEST MODE OF CARRYING OUT THE INVENTION

With reference to FIGS. 1 and 2, a head for the ejection of droplets ofliquid, also called ejection head in the following, or ejection device,or more simply ejector, made according to the method of this invention,is generically depicted with the numeral 10, and comprises a substrate11, also called actuation support, which bears at least one actuator 15,also called in the following ejection actuator; a nozzle plate 12, alsocalled orifice plate, which is provided with at least one nozzle 13 andis permanently connected to the substrate 11 along a junction zone 25;and a hydraulic circuit 21, arranged inside the head 10, the function ofwhich is to contain and convey a liquid 14 in the zone 10 between theactuator 15 and the nozzle 13, in such a way that they are both wettedby the liquid 14.

The ejection head 10 is permanently attached along the substrate 11 on acarrier 30. The actuator 15 is positioned, along the substrate 11, in azone adjacent to the nozzle 13, and is suitable for periodicallyactivating, in the volume of liquid 14 that separates it from the nozzle13, a wave of pressure, or in general a pumping effect, such as to causethe emission of a plurality of droplets 16 formed by the liquid 14,through the nozzle 13.

To this end, the actuator 15 is arranged for being driven directly bymeans of suitable electric signals or pulses, each corresponding to anejected drop, which are controlled by an electronic control unit 19 ofthe ejection head 10.

The actuator 15 may also be associated with actuation circuits, arrangedbetween the actuator and the control unit 19, which, under the controlof the control unit 19, have the specific function of generating thepulses which directly control the actuator 15 for generating thedroplets 16.

In FIG. 1, the line 18 schematically represents the electricalconnection, between the control unit 19 and the actuator 15, thefunction of which is that of transmitting the signals intended forcommanding the actuator 15 to cause ejection of the droplets 16.

In particular, the hydraulic circuit 21 comprises a first inlet duct 24,for conveying the liquid 14, which extends through the substrate 11; asecond inlet duct 22 which is formed in the nozzle plate 12 and which isin communication with one end of the first duct 24; and at least onechamber 20, also formed in the nozzle plate 12, which is adjacent toboth the actuator 15 and the nozzle 13.

The chamber 20 is suitable for being fed with the liquid 14 through theinlet duct 22, and defines an internal space in which the liquid 14 issubjected to the wave of pressure generated by the actuator 15 for beingejected through the nozzle 13.

In addition, the ejection head 10 is associated with a tank 17,containing a certain quantity of liquid 14, which constitutes a reservefor the liquid 14 to be fed to the chamber 20 of the ejection head 10,and which for this purpose is in communication with the hydrauliccircuit 21, through a feeding duct 23.

In this way, the ejection head 10 can receive the liquid 14 continuouslyfrom the tank 17, so that it is ejected in the form of droplets 16towards the outside of the ejection head 10 through the nozzle 13.

The technologies used for generating in the liquid 14 theabove-mentioned pumping effect which results in ejection of the droplets16 of liquid may be of various types and be based on differentprinciples. For simplicity's sake, in this description, reference willpreferably be made to the bubble type ejection technology, widely knownand used in the sector of printers, which is based on the generation bythe actuator 15, in the zone of the nozzle 13, of a micro bubble ofliquid vapour which, on expanding, causes the ejection of a droplet ofliquid through the nozzle 13. Clearly, however, the description thatwill be given must not be seen as tending to limit the scope of thisinvention to this particular liquid droplet ejection technology.

For instance, by way of alternative to the bubble technology, thepumping effect for ejection of the droplets could be obtained from thedeformation of a piezoelectric type actuator.

This much said, in the bubble technology mentioned, the actuator 15consists of a resistor which, in practice, is driven by the control unit19 with a brief current pulse sufficient to determine, by the jouleeffect, a rapid heating of the same resistor 15.

Accordingly the liquid 14 arranged in the immediate vicinity of theresistor 15 is brought to evaporation, and therefore causes theappearance of a vapour bubble, derived from the liquid 14, which byexpanding exerts a pumping effect in the direction of the nozzle 13 todetermine, through the latter, the ejection of a droplet 16

Then, at the end of the pulse, on account of the simultaneous cooling ofthe resistor 15, the vapour bubble collapses, so that the liquid 14adjacent to the resistor 15 returns to its starting conditions, and theresistor 15 can once again be activated with a new pulse to cause theejection of a new droplet 16. In short, this cycle is repeatedperiodically, driving the resistor 15 with a predetermined succession ofpulses which result in the generation of a like number of vapour bubblesadjacently to the resistor 15, and the ejection of correspondingdroplets 16 through the nozzle 13.

As illustrated in FIG. 1, the nozzle 13 is arranged to the front withrespect to the resistor 15, so that the expansion of the vapour bubbleis used in the normal direction to the resistor 15 to eject the droplet16. This disposition, as already said, is often called “top shooter”type, and is typical of an important category of ejection heads whichare based on the bubble technology. However the relative dispositionbetween the ejection actuator and the nozzle may also be different fromthat shown in FIG. 1, without departing from the scope of thisinvention.

As described in detail later, the liquid 14 used on the ejection head 10for being ejected in the droplet form may also be of different types,and have completely different compositions from one type of liquid tothe next, depending on the specific sector in which the ejection head 10is applied, and therefore of the specific characteristics that theliquid must possess in relation to that given sector. The nozzle plate12 and the substrate 11 constitute the essential parts of this ejectionhead 11, and are produced in two distinct processes, indicated in FIG. 2with the numerals 31 and 32 respectively, before subsequently beingassembled and connected permanently together, during a step 33, in orderto form the ejection head 10.

For clarity's sake, the two manufacturing processes 31 and 32,respectively of the nozzle plate 12 and of the substrate 11, will bedescribed separately, starting with that of the nozzle plate 12.

With reference to FIG. 3, this process comprises an initial step,represented in section (a) of FIG. 3a, wherein a wafer of silicon 51,having two opposite faces indicated respectively 51 a and 51 b, is stuckusing an adhesive substance on a carrier 52, for example on the side 51b.

The wafer 51 may readily be found in commerce and has a standard shape,for example round shape having diameter 3″ and approximate thickness 75μm.

The carrier 52 too may consist of a known type wafer, even ifconsiderably thicker than the wafer 51 used to make the nozzle plate 12.

For example the carrier 52 may be made of a round wafer of diameter 4″,thickness 0.5 mm, either of standard silicon type, or of glass orceramic.

The wafer 51 is oxidised on the outside, so as to present on its twoopposite faces, 51 a and 51 b, a thin layer 55 silicon dioxide SiO₂, ofthickness 0.3÷0.4 μm for example.

After being mounted on the carrier 52, the wafer 51 is covered in aknown way, on its free face 51 a opposite that 51 b stuck on the carrier52, with a thin layer 53 of a light-sensitive substance, called“photoresist”, 1-3 μm thick.

In particular the photoresist constituting the layer 53 is positivetype, i.e. it is such as to be, under normal conditions, resistant andnot subject to attack from certain substances, and as to become, on theother hand, easy to dissolve and remove by these substances, if exposedto light radiation.

According to known techniques and as illustrated in FIG. 3a—section (b),after application on the wafer 51 this layer 53 of positive photoresistis subsequently illuminated with light 49 coming through a suitable mask50 having a given configuration which corresponds to the positive imageof those parts of the hydraulic circuit 21, namely the inlet duct 22 andthe chamber 20, that will be formed in the nozzle plate 12.

In this way, the layer 53 is impressioned in such a way as to becomeremovable in the subsequent operation only in the areas illuminated bythe light 49.

Conveniently, for the purpose of reaching economies of scale andimproving the efficiency of the production process, the wafer 51 can beused for manufacturing a plurality of nozzle plates 12, eachcorresponding to an elementary area of the wafer 51.

To this end, the mask 50 is arranged with a configuration which is madeup of a plurality of equal profiles, each reproducing a hydrauliccircuit 21 to be made on a corresponding elementary area of the wafer51. Accordingly the positive photoresist 53 is illuminated through themask 50, and therefore becomes removable, along a plurality of equalzones, one for each elementary area of wafer 51, which correspond to theprofiles of the mask 50.

For simplicity's sake, FIG. 3a—section (b), as also the following ones,refer to and represent the structural changes which occur only in oneelementary area of the wafer 51, though it will be clear that what isdepicted in each of these figures is to be considered as repeatedexactly in each of the other elementary areas of the wafer 51.

Therefore, using known techniques, the layer 53 of photoresist isdeveloped, removing therefrom the zones impressioned by the light andaccordingly non-resistant, in order to uncover, in correspondence withthese zones, the underlying layer 55 of SiO₂, as illustrated in FIG.3a—section (c).

Later, the wafer 51 is subjected to an etching operation, the object ofwhich is to remove, in correspondence with the areas not protected bythe upper layer 53 of photoresist, the surface thickness 55 of SiO₂, inorder to uncover the underlying silicon part.

Typically this etching operation to remove the SiO₂ is effected in aliquid bath, or at any rate in a humid environment, and accordingly isalso often called “wet etching” or “wet”. Then the external layer 53 ofphotoresist is removed. In this way the layer 55 of SiO₂ forms theprotective mask for the successive operation of etching the siliconconstituting the wafer 51.

According to a variant of the process described up to now, the startingwafer may be exempt, on its faces, of the surface layer of SiO₂, andtherefore consist solely of pure silicon. In the latter case, the layerof photoresist is deposited directly on the silicon of the wafer andsubjected to the same operations of illumination, development, andremoval already described in relation to the previous case of the waferwith oxidised surface, in order to form a protective mask for thesubsequent step of etching the silicon of the wafer, which is exactlyequivalent to that performed through the layer of SiO₂, relative to theearlier case. For simplicity' sake, only the case of the wafer 51provided with the two surface layers of SiO₂ is depicted in FIG. 3.

In both the cases described above, after formation of the protectivemask for the silicon of the wafer 51, as said, either through the layerof SiO₂, or through a layer of photoresist, the wafer 51 is subjected toone or more further etching operations, which have the purpose ofselectively removing the silicon of the wafer 51 down to a given depth,in order to form the chamber 20 and the inlet duct 22, of the hydrauliccircuit 21, which are present on the nozzle plate 12.

This etching step, shown in FIG. 3a—section (d), is performed by meansof appropriate equipment in a vacuum environment, where the wafer 51 issubject to the action of agents in the gaseous or plasma state whichcombine with the non-protected silicon of the wafer 51, corroding it andremoving it down to the desired depth.

Therefore, by contrast with the etching step previously referred andperformed in a humid environment, or “wet etching”, this etching step isoften referred to as “dry etching”.

For example, in this step the wafer 51 is hollowed for a depth ofapprox. 10□□25□m, in order to form a recess 54 made of two portions 54 aand 54 b, corresponding respectively to the chamber 20 and to the inletduct 22, in which the portion 54 a has a roughly square plan shape.

Subsequently, a thick layer 56 of negative photoresist, consisting forinstance of SU8 type negative photoresist, from the name of itsproducer, is deposited, in a known process, along the entire extensionof the unstuck side 51 a of the wafer 51, in order to completely coverthe recess 54 as well. Indicatively this layer 56 is approximately 15÷30μm thick, permitting it to cover the step defined by the recess 54.

It is emphasised that this negative photoresist constituting the layer56 has the opposite behaviour to that of the positive photoresistconstituting the previous layer 53, and therefore under normalconditions it may melt in contact with certain substances, whereas, ifilluminated, it acquires a certain resistance to these substances.

Then, as illustrated in FIG. 3b—section (e), this thick layer 56 isilluminated, through a given mask 59, so as not to receive the light 49in correspondence with that portion of the same layer 56 indicated withthe numeral 58 and having a square shape in plan view, which fills theportion 54 a of the recess 54, corresponding roughly to the chamber 20.

Later, as illustrated in FIG. 3b—section (f), the layer 56 of negativephotoresist is developed and hollowed, using known techniques, in orderto remove the non-illuminated portion 58 and thereby delimit, along thebottom of the recess 54, adjacent to the chamber 20, a confined area 61,of square shape and not protected by the layer 56, corresponding to thezone of the nozzle 13 that will be formed.

At this point, as illustrated in FIG. 3b—section (g), the wafer 51 issubjected to another etching process, the object of which is to hollowthe silicon of the wafer 51 only in correspondence with the confined,square area 61, defined on the bottom of the recess 54.

This is a wet etching, being performed in a damp environment for exampleusing a compound such as KOH, and is also called anisotropic, as it isdeveloped on the crystallographic axes of the silicon constituting thewafer 51.

In particular, this etching causes the formation of a blind hole 62, ofpyramid shape, as illustrated in the plan view of FIG. 3b—section (g).

In greater detail, taking into account the side of the uncovered squarearea 61, of the thickness, of approximately 50 □m, of the silicon wallto be etched, and of the incline, of roughly 54° of the crystallographicaxes of the silicon, the etching is conducted in such a way as to formin the wall a pyramid-shaped blind hole 62, leaving a thin residuallayer of silicon, indicated with the numeral 60, at the bottom of theblind hole 62.

At this point, after the thick layer 56 of photoresist has been removed,the wafer 51 is unstuck, along the side 51 b, from the carrier 52,cleaned and then stuck again, this time on the opposite side 51 a of thesame carrier 52 or on another similar carrier.

Subsequently, as illustrated in FIG. 3b—section (h), the wafer 51 iscovered on the side 51 b, now free, with a layer 57 of positivephotoresist, represented with the dot and dash line, which is laterilluminated with a suitable mask, impressioned and developed, with thesame techniques as already seen earlier, in such a way as to protect theentire extension of the layer 55 of silicon dioxide SiO₂ arranged-alongthe side 51 b, with the exception of a limited circular area adjacent tothe wall 60 and corresponding to the nozzle 13.

The wafer 51 is then subjected to another “wet” etching process, i.e. ina chemical bath, to remove the circular, unprotected area of the layer55 of silicon dioxide SiO₂, and uncover an underlying and correspondingcircular zone of the silicon of the wafer 51.

In this way, the layer 55 forms a protective mask for the silicon of thewafer 51 during the subsequent dry etching operation.

Naturally if originally the wafer 51 was not provided with the layer ofSiO₂, this protective mask is made with a layer of photoresist, in thesame way as already seen earlier.

In particular, in this case, the layer of photoresist is selected with asuitable thickness, in relation to the thickness of silicon to be etchedin the following step, to permit a correct conduction of this etchingstep.

Then, in a dry type etching process, the circular uncovered area of thesilicon of the wafer 51, i.e. not protected by the layer 55, is etched,in such a way as to hollow the wall 60 and form in it a pass-throughhole 63 corresponding to the nozzle 13.

Finally the wafer 51 which, it will be recalled, has undergone theoperations described earlier for each of its elementary areas, is cutinto single units corresponding to these areas, and each constituting anozzle plate 12.

Following this, the single nozzle plates 12 are washed and inspected tocheck that they do not contain defects, and that they have been formedcorrectly In this way, from the wafer 51, the structure is obtained thatconstitutes the nozzle plate 12, which is shown in FIG. 3b—section (i),both in lateral section and in plan view.

The process 32 for manufacturing the substrate 11 in large part followsa known sequence and employs technologies that are also known, and willnot therefore be described in detail.

It is recalled simply that this process 32 starts with the availabilityof a carrier or wafer of silicon 70, similar to the one used formanufacturing the nozzle plate 12, but of significantly greaterthickness, for example 0.5 mm, and has the object of making on thecarrier 70, as well as the actuator 15, certain protective layers havingthe function of protecting the actuator 15 itself so as to prolong itsworking life.

In the process 32, a suitable track, or tracks, are also made, for theelectric connection of the actuator 15 with the circuits arranged fordriving it.

In particular, as anticipated above, the process 32 may also include theproduction, on the silicon wafer 70, of specific auxiliary circuits,often called “drivers”, suitable for being conditioned by the controlunit 19 for generating the pulses to be sent directly to the actuator 15for activating ejection of the droplets 16.

In the same way as the nozzle plate 12, and with the purpose of creatingeconomies of scale and improving the efficiency of the productive cycleof the substrate 11, a single wafer of silicon 70 may be used tosimultaneously produce a plurality of substrates 11, each identical andcorresponding to an elementary area or portion of the original siliconwafer 70.

For clarity's sake, the structure of the substrate 11 which is producedvia the known operations mentioned above and which corresponds to anelementary portion of the wafer 70 is represented in FIG. 4—section (a).

In particular, this structure comprises a base layer 71 of siliconcorresponding substantially to the thickness of the initial startingwafer 70; a zone 72, made in MOS technology, which comprises a series ofcircuits or drivers for controlling operation of the ejection head 10; athin layer 73 of silicon dioxide SiO₂ selectively grown on the layer ofsilicon 71, and in particular lacking along the zone 72 with the MOScircuits; a thin resistive film of limited extent or resistor 74,constituting the actuator 15; one or more tracks, not shown on thedrawings and extending in the normal direction to the plane of FIG. 4,for electrically connecting the resistor 74 to the circuits of the zone72; a protective layer 76 made of silicon nitride and silicon carbideand deposited on the resistor 74; and a layer 77, made of tantalum Ta,arranged over the nitride/carbide layer 76 in the area of the resistor15.

The layer 77 of Ta has essentially the function of protecting theresistor 74 against wear caused by the mechanical stresses to which theresistor 74 is subjected, during operation of the ejection head 10.

Typically these stresses are caused by the phenomenon of cavitation thatoccurs due to the pumping effect of the liquid 14, caused by theresistor 74, for ejecting the droplets 16.

As will be seen more clearly below, this layer 77 of tantalum isarranged for also being used advantageously during the successiveoperation of joining the substrate 11 with the nozzle plate 12, to formthe ejection head 10, and to this end the layer 77 of tantalum isdeposited on the silicon wafer 70 in order to cover not only the area ofthe resistor 74, but to extend laterally along the zone where thejunction will be made.

Also, to this same end, the layer 77 is formed in such a way as to have,along its edge, a portion 77 a, which is disposed externally withrespect to the junction zone Differently from the known art and with thepurpose of arranging the substrate 11 for the next operation, describedbelow, of-joining with the nozzle plate 12, the structure of thesubstrate 11 also comprises, along given junction zones, an outersurface layer 78 of borosilicate glass, deposited on the layer 77 oftantalum.

As illustrated in FIG. 4—section (b), this layer 78 of borosilicateglass is initially deposited continuously on all the areas of theoriginal wafer 70, in order to completely cover the layer 77 of tantalumprovided on these areas.

More particularly, the layer 78 is of a thickness of between 1÷5 μm, andis made of Pyrex 7740, or Schott 8329 borosilicate glass, containingions of sodium and lithium, with thermal expansion coefficient of2.3*10⁶K⁻¹ and therefore very close to that of the silicon which is of2.3*10⁶K⁻¹.

Accordingly the layer 78 of borosilicate glass and the silicon of thewafer 70 mate together optimally without causing the occurrence ofmechanical stresses in the junction area.

Deposition of the outer layer 78 of borosilicate glass on the substrate11 is performed in a known way, for instance by way of the process knownas “RF sputtering”, in which the borosilicate glass is atomized andsprayed on the substrate 11.

The layer 78 may also be deposited by way of the process known as“electron-beam evaporation”, in which an electronic ray is radiated uponan electrode consisting of borosilicate glass, so that the borosilicateglass evaporates and is deposited on the substrate 11.

With respect to sputtering, the electron-beam evaporation process hasthe advantage of being faster, i.e. of being able to deposit a greaterquantity of material per unit of time, and in addition of being able toensure a greater stechiometric control of the deposited layer 78 ofborosilicate glass.

This continuous layer 78 of borosilicate glass is then etched with knowntechniques in order to uncover the area of the resistor 74, and torestrict the layer 78 to the area of the substrate 11 intended forcoupling with the nozzle plate 12.

In this way, the layer of borosilicate glass 78 forms a kind of framearound the resistor 74. To this end, the continuous layer 78 is firstcovered with a layer of positive photoresist, which is then selectivelyilluminated, and finally removed in correspondence with the illuminatedzones, in order to define a protective mask for the underlying layer 78.

Later, again with known techniques and for instance by way of a dryetching step, the layer 78 of borosilicate glass is removed along theareas not protected at the top by the photoresist.

Accordingly the structure depicted in FIG. 4—section (c) and whichconstitutes the substrate 11 is obtained.

Naturally, where a single original wafer 70 is used to produce numeroussubstrates 11, this structure is duplicated into the various elementaryareas of the silicon wafer 70.

In short, this structure comprises by way of example a residual layer 78a of borosilicate glass, which is obtained from selective etching of theoriginal continuous layer 78 and is disposed laterally with respect tothe resistor 74, in order to uncover the portion of the layer 77 oftantalum which protects the resistor 74, and to also define a junctionor soldering surface 79 for the coupling of the substrate 11 with thenozzle plate 12.

In order to ensure the best results during the subsequent step ofjoining the substrate 11 with the nozzle plate 12, step which is carriedout by means of the anodic bonding technology as will be described indetail below, preferably the layer 78 of borosilicate glass is subjectedto a planarization operation along the free surface intended forcoupling with the nozzle plate 12.

The object of this operation is to reduce to a minimum roughness of thesurface of the layer 78 and it is carried out, for instance, using aplanarization process called CMP, or “Chemical-Mechanical Polishing”.

In fact, as is known, the anodic bonding process requires an exceptionaldegree of planarity of the surfaces that have to be coupled by means ofthis process.

Unfortunately the wafer 70, during the operations for forming thesubstrate 11, which precede the depositing of the layer of borosilicateglass 78, inevitably acquires a certain degree of roughness, which thesame layer 78 of borosilicate glass necessarily reproduces andamplifies.

Therefore the CMP planarization process has the object of remedying thisprogressive increase in roughness of the wafer 70, ensuring a very highdegree of planarity of the surface of the layer 78 of borosilicate glassintended for contact coupling with the nozzle plate 12.

In particular, this CMP process may be carried out following applicationof the continuous layer 78 of borosilicate glass, and before its etchingto define the residual layer 78 a and the corresponding junction surface79.

As anticipated above, and according to a characteristic of thisinvention, the plate 12 with the nozzle 13 and the substrate 11, afterbeing manufactured separately from one another as described earlier, arejoined permanently in a joining process based on the anodic solderingtechnology, frequently also called “anodic bonding”.

For information, it is pointed out that anodic bonding constitutes ajoining technology which has been developed and perfected in recentyears, and which at present is being applied to an ever greater extentin numerous sectors of the art, in particular in the field ofmicrostructures, also abbreviated MEMS standing for “MicroElectroMechanical Systems”, for the purpose of achieving a stable andefficacious junction between two parts making up a microstructure.

For instance this joining technology based on anodic bonding is used toadvantage to structurally join together two silicon wafers, in whichcase it is also known as “silicon-to-silicon anodic bonding”.

As is known, the anodic bonding technology is employed to join twosurfaces having a high degree of planarity, and is based essentially onthe principle of putting the two surfaces to be joined into reciprocalcontact at a suitable pressure and temperature, and of then applying acertain potential to them.

In this way, in fact, the junction zone becomes the seat of opportuneelectrostatic charges tending to reciprocally attract and co-penetratethe molecules of the two surfaces, so as to produce a structuralcohesion between the two.

Often this technology requires that the surfaces intended to be contactcoupled be adequately prepared, for instance by means of depositing onat least one of them a suitable layer of material.

Further, as already said, this technology also requires the two surfacesto be coupled to be extremely flat and without roughness, i.e. matingperfectly along the zone of contact, so that the phenomenon ofco-penetration and structural cohesion between the respective moleculescan take place.

Further details and information about the anodic bonding technology maybe obtained in the following publications, quoted below by way ofreference:

“Field Assisted Glass-Metal Sealing”, published on page 3946, of volume40, No. 10, Sep. 1969, of the magazine “Journal of applied physics”;

“Fabrication of a silicon-Pyrex-silicon stack by a.c. anodic bonding”published on page 219 et seq, of No. A 55, 1996, of the magazine“Sensors and Actuators”;

“Anodic bonding technique under low temperature and low voltage usingevaporated glass”, published in Vol. 15, No. 2, March/April 1997, of themagazine “Journal of Vacuum Science Technology”;

“Silicon-to-silicon wafer bonding using evaporated glass”, published onpage 179 et seq, of No. A 70, 1998, of the magazine “Sensors andActuators”.

For completeness, FIG. 5 schematically represents the step of joiningthe nozzle plate 12 with the substrate 11 using the anodic bondingtechnique, and the anodic bonding equipment or machine, genericallyindicated with the numeral 85, used to make the junction.

In particular, the anodic bonding equipment 85 comprises twocounter-electrodes, generically indicated with the numerals 81 and 82,adapted for working respectively as the anode and the cathode in theanodic bonding step. In detail, initially the nozzle plate 12 and thesubstrate 11 are arranged in reciprocal contact on the smooth surface 79defined by the layer of borosilicate glass 78 a, and in addition alignedwith precision with respect to one another. Thus, during a punchingoperation, the nozzle plate 12 and the substrate 11 are temporarilyconnected one to the other, for instance with a laser ray, or by meansof a suitable adhesive, so that they are held together, at least untilthe definitive junction is made. Then the assembly formed by the nozzleplate 12 and the substrate 11 is loaded on the anodic bonding machine85, setting the substrate 11 on a heating element 83 the object of whichis to heat and maintain the substrate 11 at a temperature between 200and 400° C., during the anodic bonding.

Moreover, the assembly formed by the nozzle plate 12 and the substrate11 is disposed on the bonding machine 85 setting the anode 81 of thelatter on top of the nozzle plate 12, with a certain pressure, and alsoelectrically connecting the cathode 82 of the anodic bonding machine 85with the portion 77 a, of the tantalum layer 77, which extends to theoutside of the zone of contact between the substrate 11 and the nozzleplate 12. In particular, the anode 81 is plate-shaped so as topractically cover the nozzle plate 12 over its entire extent.

The cathode 82 of the bonding machine 85 is also connected to the mainlayer of silicon of the substrate 11, and to the heating element 83, tokeep them at the same potential during the bonding operation. At thispoint, the anodic bonding machine 85 applies, for instance during aperiod of 15 minutes, a potential defined by a voltage V, ofindicatively between 50 and 500 volt, between the anode 81 and thecathode 82, thus activating that phenomenon called, as already stated,anodic bonding which gives that structural cohesion between theborosilicate glass of the layer 78 a and the silicon dioxide SiO₂ on thesurface of the nozzle plate 12.

As tantalum is conductive, the layer 77 operates in this anodic bondingstep as a cathode plate true and proper which distributes the potentialdifference generated by the anodic bonding machine 85 through thejunction zone, so that the bonding assumes uniform characteristics overits full extent.

Accordingly the substrate 11 and the nozzle plate 12 are joinedpermanently and structurally through a junction, indicated with thenumeral 25 which extends along a corresponding junction zone defined bythe layer 78 a of borosilicate glass deposited on the substrate 11.

In this way, the ejection head 10 is formed, with the relative internalhydraulic circuit 21 intended for conveying the liquid 14 inside theejection head 10.

The ejection head 10 manufactured in the above way with the junction 25presents numerous and important innovative aspects with respect to theknown way.

First and foremost, unlike what happens in the known art, the substrate11 and the nozzle plate 12 of the ejection head 10 are bound closelytogether in a joining process that does not involve the use ofadditional substances, such as binders or other compounds, generally ofthe organic type, liable to cause a certain structural discontinuity inthe junction zone.

In fact, the anodic bonding technology, via which the junction 25 isproduced, is characterized precisely by its ability to produce acomplete continuity and structural co-penetration between the materialsof the parts that are being joined, in the specific case between thesilicon of the nozzle plate 12 and the borosilicate glass deposited onthe substrate 11.

In particular, the structure of the ejection head 10 obtained throughthis method does not present, either in the parts that comprise it, oron the junction 25, organic type substances, or other similar materials,so that the ejection head 10 can advantageously be employed, withoutsuffering damage, such as for instance corrosion, and/or unsticking,which would compromise its operation, even with liquids that areespecially aggressive vis-a-vis organic compounds.

As a general concept, it may be said that the ejection head 10 of theinvention is characterized by the fact of comprising, between the nozzleplate 12 and the substrate 11 bearing the ejection actuator 15, ajunction 25 which has the property of being substantially inert from thechemical point of view.

In other words, this junction 25, in relation with the liquid 14contained in the hydraulic circuit 21 of the ejection head 10 andthereby wetting the zone of the same junction 25 in being ejected indroplet form by the ejection head 10, possesses special properties ofresistance to chemical corrosion by the liquid 14, and also of noncombining chemically with the latter, which are at least equal andequivalent, and at any rate not inferior, to those of the materials, inparticular silicon, and/or of the parts that comprise the structure ofthe nozzle plate 12 and of the substrate 11, and which are also wettedby the liquid 14.

Description of a First Example of Application of the Invention forProducing an Ink Jet Printhead

FIG. 6 shows in section view an ink jet printhead, indicated genericallywith the numeral 110 and suitable for being fed with ink 140, which isproduced in accordance with the method of the invention. Where possible,the parts of the printhead 110 corresponding to those of the ejectionhead 10 are indicated with reference numerals incremented by 100 withrespect to the ejection head 10.

In particular, the printhead 110 comprises a nozzle plate 112 and asubstrate 111, also called “die”, which are made separately from oneanother and then joined permanently together via a junction 125, in asimilar way to the manufacturing process described in connection withthe ejection head 10. More particularly, the junction 125 ismanufactured with the anodic bonding technology, after appropriatelypreparing the substrate 111 by depositing on it a layer 178 ofborosilicate glass.

The substrate 111 and the nozzle plate 112 define a plurality ofejection units, indicated with numerals 110 a, 110 b, 110 c, etc., whichare arranged along an ejection side 150 of the printhead 110 and have,each one, a structure corresponding to that of the ejection head 10.

Each ejection unit 110 a, 110 b, 110 c, etc., comprises a respectivenozzle, indicated in order with numerals 113 a, 113 b, 113 c, etc., arespective actuator 115 a, 115 b, 115 c, etc. and a respective ejectionchamber 120 a, 120 b, 120 c, etc.

The printhead 110 is also provided internally with a hydraulic circuit121 the function of which is to feed the ink 140 from a single tank 117to the different ejection units 110 a, 110 b, 110 c, etc., and whichcomprises, in addition to the chambers 120 a, 120 b, 120 c, etc., aplurality of inlet ducts 122, each communicating with a respectiveejection chamber 120 a, 120 b, 120 c, etc., and a central slot 123 madethrough the substrate 111.

In particular, the central slot 123 communicates at one end with thetank 117, and at the opposite end with the plurality of inlet ducts 122,which in turn are arranged both on one side and the other of the slot123 in order to put the slot 123 in communication with the ejectionchambers 120 a, 120 b, 120 c, etc. of the different ejection, units 110a, 110 b, 110 c, etc.

In this way, the ink 140 can flow from the tank 117 to each singleejection unit 110 a, 110 b, 110 c, etc. through the hydraulic circuit121. As already intimated, the method for manufacturing the printhead110 is substantially similar to that for manufacturing the ejector 10.

Again in this case, with a view to improving efficiency oft theindustrial mass production of these printheads 110, a single siliconwafer may be used in order to produce multiple substrates 111 and alsoto produce multiple nozzle plates 112, with obvious advantages in termsof industrial production at lower costs.

In detail, as shown schematically in FIG. 7, multiple nozzle plates 112,corresponding to elementary portions 112 a, 112 b, 112 c, etc., of anoriginal silicon wafer 151, are produced together on the originalsilicon wafer, in the steps described with reference to the nozzle plate12, so as to form for each nozzle plate 112 the respective ejectionchambers 120 a, 120 b, 120 c, etc. and the respective nozzles 113 a, 113b, 113 c, etc.

Finally, in accordance with what is indicated by the arrow 160, thiswafer 151 is cut or singularized into units each of which constituting anozzle plate 112.

Similarly and as illustrated in FIG. 8, multiple substrates 111, eachcorresponding to an elementary portion 111 a, 111 b, 111 c, etc., of asingle original silicon wafer 170, are simultaneously formed on thelatter in the steps already described with reference to the substrate11.

In particular, these elementary portions or areas 111 a, 111 b, 111 c,etc. of the silicon wafer 170 are subjected to a series of operations inorder to produce, in correspondence with each of these, a structure ofthe type depicted in FIG. 4—section (c), with a layer of borosilicateglass 178 defining a junction zone for the next anodic bondingoperation.

Conveniently, for the purpose of preparing the silicon wafer 170 for thesubsequent joining operation with anodic bonding, the conductive layersof tantalum in the areas 111 a, 111 b, 111 c, etc are interconnected toone another and to a conductive ring 177 a made along the edge of thewafer 170, so as to form, on the surface of the wafer 170, a mesh 177,also called equipotential mesh or network on account of its ability tokeep the elementary areas 111 a, 111 b, 111 c, etc. at a same potentialduring joining with the nozzle plates 112.

An equipotential network of the type of the mesh 177 is described in theItalian patent application No. TO99A000987, filed on Nov. 15, 1999 onbehalf of the Applicant, the said application being cited here forreference for all details, not found in this description, of theconfiguration and characteristics of the mesh 77.

In this way, the silicon wafer 170 acquires a structure whichencompasses a plurality of elementary areas 111 a, 111 b, 111 c, etc.,each corresponding to a substrate 111, which are already prepared forjoining with the respective nozzle plates 112.

Then the single nozzle plates 112 which, as already said, have been madeseparately, are mounted, aligned, and temporarily affixed, one by one,on the different elementary areas 111 a, 111 b, 111 c, etc., defined onthe silicon wafer 170 and therefore still permanently interconnected toone another. At this point, it is possible to proceed with the anodicbonding step true and proper, in which each nozzle plate 112 is joinedwith the corresponding elementary area 111 a, 111 b, 111 c, etc. of thesilicon wafer 170, by applying a given potential between the same usingan appropriate anodic bonding machine.

In order to permit a correct locating of the anode on the differentnozzle plates 112 and therefore optimal bonding thereof with therespective areas 111 a, 111 b, 111 c, etc. of the silicon wafer 170,this anodic bonding machine has a specially modified anode, divided inparticular into a plurality of elements, each corresponding to a nozzleplate 112, which are mounted on a sprung structure that permits limitedmovements between one anode element and another.

In fact, in this way, each of these anode elements is capable ofadapting, independently from the others, to the corresponding nozzleplate 112, so as to set perfectly on the latter with the right pressure,when the anode of the anodic bonding machine is brought globally intocontact against the various nozzle plates 112.

In turn, the cathode of the bonding machine is brought into contact,possibly at numerous points, with the outer conducting ring 177 a, towhich the various layers of tantalum, forming the mesh 177 and arrangedon the elementary areas of the silicon wafer 170 are connected.

In this way, all these layers of tantalum are brought to and maintainedat the same potential, in the anodic bonding step.

In particular, this anodic bonding step consists, as stated earlier, inputting into reciprocal contact at a given pressure and temperature eachnozzle plate 112 with the respective area 11 a, 111 b, 111 c, etc. andin applying a suitable potential between them, through the anode whichpresses with its elements on each nozzle plate 112, and the cathodewhich is connected via the mesh 177 to the tantalum layers arranged oneach area 111 a, 111 b, 111 c, etc.

Accordingly, that close structural cohesion, typical of the anodicbonding technology, is achieved between each nozzle plate 112 and thecorresponding elementary area 111 a, 111 b, 111 c, etc. of the siliconwafer 170.

Finally, after the junction has been made, the silicon wafer 170 is cutor singularized into single blocks, each of which formed by a nozzleplate 112 and a substrate 111 permanently and structurallyinterconnected, and constitutes an ejection assembly suitable for beingsubsequently assembled with a tank for forming a printhead 110 such asthe one shown in FIG. 6.

The method of the invention can be used for producing a printheadcapable of working with inks decidedly more aggressive than thoseneutral ones, generally water or alcohol based, used on traditional inkjet heads. In fact, the so-called aggressive inks, while fully innocuousin relation to the head of the invention, are capable, if used withtraditional printheads, of irreparably damaging the structure in a veryshort time, particularly in the junction zone or zones between the partsthat comprise the traditional printheads, these junctions, as is known,being made with substances easily attacked by and/or combinable withthese aggressive inks. Furthermore, this method which adopts the anodicbonding technology has the additional advantage over the traditionalmethods of involving the occurrence of lesser heat expansions and ingeneral lesser deformation during the joining step between the nozzleplate and the substrate, both of silicon, in forming the ink jetprinthead.

On the contrary, with the traditional method, the nozzle plate and thesubstrate, as also the hydraulic circuit are normally made of differentmaterials, such as for example: metal, silicon, and plastic, so thatthese parts, when connected together to form the printhead, may giverise to reciprocal deformations likely to have a negative influence onmanufacturing precision of the printhead.

Therefore, in short, the method of the invention enables compliance tobe guaranteed with extremely low manufacturing and assembly tolerances,and accordingly decidedly much higher production precision levels to bereached than with the traditional method.

Description of a Second Example of Application of the InventionConcerning an Injector for Internal Combustion Engines

FIG. 8 illustrates schematically an application in which the ejectionhead of the invention constitutes a fuel injector for an internalcombustion engine, indicated generically with the numeral 200, andcomprising at least one cylinder 201 with a piston 202 and a combustionchamber 203; an inlet duct 204 bringing fresh air to the combustionchamber 203, and an exhaust duct 206 for the fumes from the combustionchamber 203.

For simplicity's sake, a single cylinder 201 is depicted in FIG. 9, evenif it is clear that the engine 200 may comprise multiple cylinders,according to types widely known in the art.

A valve 207 is disposed in correspondence with the outlet zone of eachof the ducts 204 and 206 in the combustion chamber 203, for the purposeof excluding or otherwise the flow of air to and the flow of fumes fromthe latter-named. The inlet duct 204 is suitable for receiving the airfrom a filter zone 208, where the fresh air is suitably filtered, andaccommodates on its inside a butterfly valve 209 with the function ofcontrolling the flow of filtered air in the direction of the arrow 205towards the combustion chamber 203.

The injector, indicated with the numeral 210, has the function ofejecting droplets of fuel, such as petrol or diesel, in the inlet duct204, in quantities controlled exactly by a control unit 211, associatedwith the ejector 210, so as to form with the filtered air coming fromthe filter zone 208 an air-fuel mix which feeds the combustion chamber203.

In particular, the optimal quantities of fuel to be injected in dropletform are determined by the control unit 211 on the basis of data sent tothe latter, on lines 212, by suitable sensors in the engine.

The injector may be mounted in the position indicated with the letter A,after the butterfly valve 209, in the case of Multipoint injection (orMPI, “Multi Point Injection”, i.e. with one injector for each cylinder;or also alternatively in the position indicated with B, before thebutterfly valve 209, in the case of Single Point injection (SPI), i.e.with a single injector generating the air-fuel mix which is then sharedbetween the cylinders. In the latter case, the air inlet duct dividesinto numerous ducts corresponding to the cylinders of the engine,immediately after the butterfly valve 209.

In this way, the injector 210 of the invention permits to dose withgreat precision the quantity of fuel delivered to the cylinder, orcylinders, of the engine, so as to obtain better performances from theengine, such as for example a higher thermal efficiency, than thetraditional engines.

Furthermore the injector has a particularly robust structure, suitablefor resisting efficaciously the system of thermal and mechanicalstresses and the corrosive actions of a chemical nature depending on thefuels used, typically present in internal combustion engines.

Other Possible Applications of the Injection Head According to theInvention

The forms of application of the ejection head manufactured in accordancewith this method are not limited to those described above.

In fact, this ejection head, by virtue of its chemically inert structurein the junction zone between the actuation support and the nozzle plate,is suitable for being used in multiple sectors which require preciseinjection of special liquids, sometimes specifically developed for thesesectors, and decidedly more aggressive from the chemical viewpoint thanthe inks, both water-based and even alcohol-based, which are usuallyemployed for printing on paper media with the conventional ink jetprintheads.

One particular example that springs to mind is the industrial markingfield in general, in which this ejection head could be used to advantagefor ejecting liquids, such as special paints or inks, capable ofadhering stably also to non-paper media, such as plastic or metalliclaminates, in order to produce particular markings on these media.

For example, the ejection head could be used for making custom images onplastic media, such as those generically designated with the word“badge”, or on numerous consumer products, such as skis, helmets, tiles,gift objects, and still others. In fact, the liquids currently used forthese marking applications, and probably also those that will bedeveloped in the future, are incompatible with use on the traditionalprintheads, since they are prepared with substances or solvents whichwould irreparably damage the structure of the traditional heads, whereason the contrary these could be employed without any drawback on thisejection head.

Purely by way of example, quoted below are some types of solvents whichalready today are of wide scale application in products such as fuels,paints and printing inks, and which could be used for preparing liquidsto be used, without drawbacks, in combination with the ejection head ofthe invention, thanks to the latter's chemically inert structure:

aliphatic and aromatic hydrocarbons such as: liquid paraffins, toluene,xylene;

aliphatic and aromatic alcohols such as: methyl alcohol, isopropylalcohol, n-propyl alcohol, sec-butyl alcohol, isobutyl alcohol, n-butylalcohol, benzyl alcohol, cyclohexanol;

esters such as: methyl acetate, ethyl acetate, isopropyl acetate,n-propyl acetate, sec-butyl acetate, isobutyl acetate, n-butyl acetate,amyl acetate, 2-ethoxy ethyl acetate;

glycol esters such as: 2-methoxyethanol, 2-ethoxyethanol,2-butoxyethanol;

ketones such as: acetone, methy ethyl ketone, methyl isobutyl ketone,methyl isoamyl ketone, cyclohexanone;

lactones such as: 6-caprolactone monomer.

Another possible application of this ejection head is that ofmicrodosing, in particular though not exclusively in the biomedicalsector. In fact, this ejection head, thanks to its chemically inertstructure without organic substances, may be used without drawbacks forejecting and dosing a vast range of liquids used in the medical field,for instance organic liquids in general and more particularly liquidscontaining urea, or liquids such as insulin, or still other medicalliquids which need to be dosed with special precision in certain medicalfunctions. Even use of this ejection head for ejecting in a controlledmanner edible liquids, i.e. foodstuffs, may be numbered among thepossible forms of application of the invention. In general, it may besaid that this ejection head has a chemically inert structure which, aswell as the advantage of not being subject to corrosion by a vast rangeof liquids used in the medical field, has the further advantage of notcombining with these liquids, and therefore of not altering andoffending even minimally the characteristics while they are maintainedin this ejection head.

It remains understood that changes and/or improvements may be made tothe method for manufacturing a head for ejecting a liquid in dropletform, as also to the ejection head manufactured in accordance with themethod, described up to this point, without exiting from the scope ofthe invention.

What is claimed is:
 1. Method for manufacturing an ejection head (10;110), or ejector, suitable for ejecting a liquid (14; 140) in the formof droplets (16), and possessing internally a hydraulic circuit (21;121) for containing and conveying said liquid (14; 140), comprising thefollowing phases: producing a nozzle plate (12; 112) having at least oneejection nozzle (13; 113 a, 113 b, 113 c); producing a substrate (11;111) or actuation support having at least one actuator (15; 115 a, 115b, 115 c) for activating the ejection of said droplets (16) of liquidthrough said at least one nozzle (13; 113 a, 113 b, 113 c); andintegrally joining said nozzle plate (12; 112) and said substrate (11;111) together to form said ejection head (10; 110) and the relativehydraulic circuit (21; 121), this joining phase comprising theproduction by means of the so-called “anodic bonding” technology of ajunction (25; 125), between said nozzle plate (12; 112) and saidsubstrate (11; 111), arranged for being wetted by said liquid (14; 140)contained in the hydraulic circuit (25; 125), wherein the phase ofproducing said nozzle plate (12; 112) includes the following steps:providing a plate or wafer (51) made of silicon, selectively removingthe silicon of said plate (51) down a given depth, so as to form, alonga face (51 a) of said plate, a recess (54) defining a chamber (20) ofsaid hydraulic circuit (21), and forming, by means of an etching processand along a bottom (61) of said recess (54), said at least one ejectionnozzle (13), wherein the phase of producing said substrate (11; 111)includes the following steps: providing a plate of wafer (70, 71) madeof silicon, forming, on an outer surface of said plate (11), said atleast one actuator (15) and the tracks (72) for the electricalconnection of it, depositing a first protective layer (76) on said atleast one actuator (15), depositing a second protective and conductivelayer (77) over said first protective layer (76), said second conductivelayer (77) being arranged in the area of said at least one actuator (15)and in the junction zone where said substrate (11) will be joinedtogether with said nozzle plate (12), and moreover forming a portion (77a) which extends, along said substrate (11), outside said junction zone,depositing a preliminary layer of glass (78) on said conductiveprotection layer (77), said preliminary layer having the purpose ofpreparing said substrate (11) for being joined with said nozzle plate(12) by means of said anodic banding technology, and subsequentlyetching said layer of glass (78) to uncover the zone of said actuator(15) and to define the junction areas (78 a) between said substrate (11)and said nozzle plate (12), and wherein the joining phase includes thefollowing steps: positioning into reciprocal contact said nozzle plate(12; 112) of silicon and said substrate (11; 111), in correspondence ofsaid layer of glass (78), in such a way to arrange exactly said at leastone nozzle (13; 113 a, 113 b, 113 c) in front of said at least oneactuator (15; 115 a, 115 b, 115 c), and affecting said junction (25)between said nozzle plate (12) and said substrate (11) by connectingsaid nozzle plate (12) and said portion (77 a) of said conductive layer(77) respectively to a first (81) and a second counter-electrode (82) ofan appropriate anodic bonding machine (85), and then applying by meansof said machine (85) a determined voltage between saidcounter-electrodes (81, 82), said first counter-electrode (81) beingformed of a plate which rests on said nozzle plate (12) along the sidebearing said ejection nozzle (13) and acts as the anode during theproduction of said junction (25), whereas said second counter-electrode(82) acts as the cathode, whereby a structural cohesion is obtainedbetween the two surfaces of silicon and of glass (78), in reciprocalcontact, respectively of said nozzle plate (12) and of said substrate(11).
 2. Method for manufacturing an ejection head according to claim 1,wherein said preliminary layer is made of borosilicate glass (78). 3.Method for manufacturing an ejection head according to claim 2, whereinsaid layer of borosilicate glass (78) is made of a material known asPyrex containing sodium.
 4. Method for manufacturing an ejection headaccording to claim 1, wherein the phase of producing said substrate (11)comprises a step of planarization (CMP) to planarize said layer of glass(78) on the free surface intended for coupling with said nozzle plate(12), said step of planarization having the task of ensuring a highdegree of planarity on said free surface for allowing said layer ofglass (78) to interface and couple at contact with said nozzle plate(12).
 5. Method for manufacturing an ejection head according to claim 1,wherein, during the phase of joining said substrate (11) and said nozzleplate (12) by means of said anodic bonding technology, said substrate(11) is maintained at a pre-established temperature by means of aheating element (83).
 6. Method for manufacturing an ejection headaccording to claim 1, wherein said actuator (15; 115 a, 115 b, 115 c) isof the thermal type and in particular is made of a resistor (74) whichis suitable for rapidly heating in order to generate, within said liquid(14; 140), a vapour bubble suitable to cause the ejection of saiddroplets, and wherein said conductive protection layer (77; 177) in madeof tantalum (Ta).
 7. Method for manufacturing an ink jet printhead (110)possessing internally a hydraulic circuit (121) for containing andconveying ink (140), comprising the following phases: producing a nozzleplate (112) having at least one ejection nozzle (113 a, 113 b, producinga substrate (111) having at least one actuator (115 a, 115 b, 115 c) foractivating the ejection of said ink (140), in droplet form, through saidat least one nozzle (113 a, 113 b, 113 c); and integrally joining saidnozzle plate (112) and said substrate (111) together to form saidprinthead (110) and the relative hydraulic circuit (121), said joiningphase comprising the production of a junction (125), between said nozzleplate (112) and said substrate (111), arranged for being wetted by theink (140) contained in the hydraulic circuit (121), wherein the phase ofproducing said nozzle plate (112) comprises the following steps:providing a plate or wafer made of silicon; selectively removing thesilicon of said plate down a given depth, so as to form, along a face ofsaid plate, a recess defining a chamber of said hydraulic circuit (121);and forming, by means of an etching process and along a bottom of saidrecess, said at least one ejection nozzle (113 a, 113 b, 113 c); whereinthe phase of producing said substrate (111) comprises the followingsteps: providing a plate or wafer made of silicon; forming, on a face ofsaid plate, said at least one actuator (115 a, 115 b, 115 c) and thetracks for the electrical connection of it: depositing a firstprotective layer of silicon nitride and of silicon carbide on said atleast one actuator, depositing a second protective and conductive layer(177) of tantalum over said first protective layer of silicon nitrideand of silicon carbide, said second conductive layer (177) of tantalumbeing arranged in the area of said at least one actuator and in thejunction zone where said nozzle in plate (112) and said substrate (111)will joined together, depositing a continuous layer of borosilicateglass (178) over said second layer (177) of tantalum, selectivelyetching said continuous layer of borosilicate glass (178) in such a waythat it extends only over said junction zone, and planarizing (CMP) thefree surface of said layer of borosilicate glass (178), so as to ensurea high degree of planarity of said surface adapted for the successivejunction phase of said substrate (111) with said nozzle plate (112), andwherein the phase of joining said substrate (111) and said nozzle plate(112) comprises the following steps: positioning into reciprocal contactsaid nozzle plate (112) and said substrate (111), in correspondence of asaid layer of borosilicate glass (78), in such a way to face exactlysaid at least one nozzle (113 a, 113 b, 113 c) to said at least oneactuator (115 a, 115 b, 115 c), temporarily connecting together saidnozzle plate (112) and said substrate (111), and joining, by means ofthe so-called “anodic bonding” technology, the assembly formed by thenozzle plate and the substrate, whereby a structural cohesion isobtained between the two surfaces of silicon and of borosilicate glass(98), in reciprocal contact, respectively of said nozzle plate (112) andof said substrate (111).
 8. Method for manufacturing an ink jetprinthead (110) according to claim 7, wherein the phrase of producing anozzle plate (112) comprises the following steps: providing a siliconwafer (151) comprising a plurality of elementary areas (112 a, 112 b,112 b) each corresponding to a nozzle plate; forming by etching, on eachof said areas, at least one chamber (120 a; 112 b; 112 c) and one inletduct (122) of the hydraulic circuit (121) of the corresponding nozzleplate (112), said inlet duct (122) being provided for feeding the ink(140) to said chamber (120 a; 120 b; 120 c); and dividing said siliconwafer into elementary units each constituting a nozzle plate (112). 9.Method for manufacturing an ink jet printhead (110) according to claim8, wherein said silicon wafer is of the thin type and has an indicativethickness of 75 μm.
 10. Method for manufacturing an ink jet printhead(110) according to claim 8, further comprising the following steps:providing a silicon wafer (170) comprising a plurality of elementaryareas (111 a, 111 b, 111 c) each corresponding to a substrate (111);providing, on said silicon wafer (170), a protection layer of conductivematerial consisting of a plurality of reciprocally interconnectedportions in such a way as to form an equipotential mesh or network(177), wherein each portion of said conductive layer is deposited on arespective elementary area (111 a, 111 b, 111 c) of said silicon wafer(170), and extends both along the area of said actuator (115 a, 115 b,115 c) for the purpose of protecting it, and along the zone of thejunction (125) which will subsequently be made between the substrate(111) and the nozzle plate (112), and in addition also externally to thejunction zone (125); providing a plurality of nozzle plates (112), madeseparately with respect to said substrate (111), aligning and arranging,on said silicon wafer (170), each of said nozzle plates (112) intocontact with a corresponding elementary area of said silicon wafer(170); connecting said equipotential network to a counter-electrode ofan appropriate anodic bonding machine; applying, by means of saidcounter-electrode, a suitable potential between said equipotentialnetwork and each nozzle plate (112) to produce said junction (125),based on the anodic bonding technology, between each elementary area(111 a, 111 b, 111 c) of said silicon wafer (170), and correspondingnozzle plate (112), and dividing said silicon wafer (170) into aplurality of units, each formed by a single substrate and a singlenozzle plate, and constituting an ink jet printhead.
 11. Method formanufacturing an ink jet printhead (110) according to claim 10,comprising, after said step of providing a plurality of nozzle plates(112) on said silicon wafer (170), a step of connecting temporarily withon adhesive each of said nozzle plates (112) to the correspondingelementary areas (111 a, 11 b, 111 c) of said silicon wafer (170).